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Aviation History
1956
1956 - 1717.PDF
7 December 1956 881 power running under the wide range of conditions experiencedby aircraft. In general, the missile turbojet is operated at con- stant r.p.m., and requires only a rudimentary fuel system anddrives few accessories. * Provided that they do not have to fly higher than about 90,000ft,faster long-range cruise missiles can be most efficiently propelled by ramjets. The modern supersonic ramjet is an attractive primemover and, since the oxidant is obtained from the atmosphere, the specific fuel consumption is only about one-fifth as great asthat of a rocket. Furthermore, a ramjet can be designed to run either on conventional jet fuel or on one of the later so-calledchemical fuels with very high heat content per unit weight or bulk. The inherent ramjet drawback of inability to start fromrest is readily overcome in a missile by employing a booster rocket. A typical flight time for a defensive ramjet-missile is 10 to15 minutes, during which time 300 miles may be covered. It is marginally possible to feed the fuel by air-bottle pressure, and itis more usual to employ a proper fuel pump and control system. Power to drive the fuel pump can be obtained either from aram-air turbine or from a self-contained powerplant fitted with appropriate automatic regulation. Two extremes of ramjet mis-siles are the American Navaho, a large and very advanced machine with intercontinental range, and a small French tactical missilewith subsonic performance, driven by a ramjet integral with the fuselage. In spite of the attraction of the above schemes, more than 90 percent of present missiles are driven by rockets. Such propulsion is invariably the most attractive where the range is less than 30miles. Rocket motors for missiles are of every conceivable size and character and include units with a greater sustained power-output than anything else yet built—not excepting even the Hoover Dam. The simplest installations employ solid propellants. By defini-tion, it is not normally possible to pump a solid propellant and it therefore has to be burned in situ. For this reason such rocketswere originally looked upon as suitable only for booster motors and other applications of short duration. Recently there has beena considerable change in policy, and modern solid rockets are finding wide acceptance in missiles with very respectable range.Moreover, both in Britain and America solid propellants are also being used for the primary propulsion of really large vehicles.Developments are also in hand to feed fuel as strip or tube from a coil. All solid propellants are finely divided mixtures of fuel andoxidant. In many cases it is possible to employ material akin to cordite. Alternatively the mixture may include a variety ofother compounds, including petroleum derivatives. Two of the prime criteria are that the filling should be as dense as possibleand burn to form the lightest possible gas with a high jet tem- perature, low molecular weight and high jet velocity. A solid rocket is not merely a case stuffed with propellant.The latter is extruded with a cross section (which may be quite complex) suitable for efficient combustion; the area of burningremains constant as the propellant is consumed, so that thrust stays roughly constant. Many years of research have precededthe evolution of the optimum shapes of filling which ignite cleanly and burn without "chugging" or other erratic behaviour. Inrecent years solid booster motors have been developed either with no casing at all or with a casing which forms part of the propellantor is otherwise consumed. Previously, falling booster cases constituted a potential hazard to friendly life and property. While combustion is taking place an equilibrium is establishedbetween the burning propellant and the resulting hot gas, which renders the process largely self-governing. Reduction of theinterior pressure slows down the rate of burning and can even cause combustion to cease; thus, by providing suitable valves inthe chamber, it is possible to make a solid motor controllable. Meanwhile, steady improvements in propellants, together withbetter motor design, have allowed the solid rocket to penetrate fields previously the preserve of the much more complex liquidmotor. This has sometimes been accelerated by inability of the liquid motors to meet requirements. A liquid rocket has to have tanks, feed lines and a combustionchamber, together with an appropriate control and safety system. When the required firing time exceeds about 30 sec it may alsobe necessary to employ a pump, which necessitates a source of shaft-power. For a motor of 10,000 1b thrust the propellantsmay have to be fed at 50/lb sec, and to make the whole system foolproof and efficient is very difficult, in view of the environ-mental conditions to which a missile is subjected. In recent years, however, much has been done to ease theproblems of complexity and unreliability which have plagued many of the earlier liquid motors. For many missile applicationsit is possible to use an uncooled chamber with a suitable type of refractory lining. Furthermore, ingenious ways of surmount-ing the problems of fuel systems are now bearing fruit. As an example it is possible to cite the British K.P.4 motor, in whichfuel is housed in cylindrical tubes and fed to the motor by free pistons moved by gas pressure. A single chamber might be fed by six tubes of liquid oxygen and one of hydrocarbon fuel.Warhead. Of all the aspects of guided weapons, that most subject to stringent security is the joint factor of warheads andfuzing. Nevertheless a few notes are admissible. Fuzing is most commonly of the proximity type. Such deviceswere first developed in 1944 and trigger the warhead either when the missile approaches within a specified critical distance of itstarget or when it reaches its nearest point. At launching the missile is "safe" and the fuzing is armed during flight. In a typicalanti-aircraft system the fuzing system emits radar signals which are reflected from the target. So long as the return signal strengthgoes on increasing, the armed warhead is left alone; but the moment the rate-of-increase of signal strength becomes zero (asit does when the missile reaches its nearest point to the target) the charge is triggered. This holds good even if the miss-distanceis as much as a mile. Of course, the warhead would also be triggered by a direct hit. The greatest allowable miss-distance depends on the size ofcharge and the accuracy of the weapon. If guidance is good (fully-active homing, for example) the required miss-distancecan be quite low—perhaps only 10 or 20ft. Conversely, the poorer the guidance, the greater must be the proximity-fuse miss-distanceand the larger must be the warhead. Apart from nuclear devices, which are a special case, most anti-aircraft missile warheads depend for their effect entirely on frag- mentation. Blast, even from a heavy charge, has a very limitedlethal radius. There is, of course, an optimum fragment size. If the splinters are too big there will not be enough of them toensure that the target is vitally hit; if too small they will lose energy too soon and fail to penetrate. It has not been found easyto achieve this optimum size. A recent American report described a relatively simple means of doing so; the warhead is wrapped inwindings composed of "optimum fragments" threaded on nylon cord. Accessory Systems. Engineers responsible for developing anycomponents required for supersonic aircraft have now learned that the problems posed by sustained supersonic flight demand awholly new approach to design. In some respects the require- ments of a missile component are less arduous. Flight-time isinvariably much shorter and, except for two or three seconds of fierce initial acceleration, the entire flight takes place at more orless constant supersonic speed. Nevertheless, most design-con- siderations are adverse: — Space: Volume must be minimized, requirements in this respectbeing more stringent than in any type of aircraft. It is rarely possible to accommodate any item larger than a 6in-cube in ashort-range weapon. Weight: This also must be minimized and integrated with theequipment disposition to produce the correct e.g. position. In the typical SAM, 5 lb of additional weight may reduce the rangeby a mile. Vibration: This may well be of extreme severity. Irrespectiveof the type of propulsion, the combination of extreme thrust, very high velocity, aeroelasticity and small overall weight can result inhigh-frequency oscillation with an amplitude clearly visible to the eye. Acceleration: To state that the whole missile has to resist 40ggives little concept of what is involved. Nobody would reasonably expect a television set to work after being thrown out of a third-floor window, but the more complex interior of a missile could withstand such treatment as a matter of course. It is interestingto note that American workers have found sled-testing invaluable. Components which pass all vibration tests have frequently beenfound to fail under sustained acceleration on a sled. Temperature: Limits of — 50 and + 300 deg C might be met bydifferent parts of a missile, but in weapons of fairly short range thermal lag may ease the extreme interior conditions. Neverthe- As described at the top of the column above, missile warheads are arranged to be detonated by a proximity fuse at the nearest point to the target of the weapon's path. Here, a Fireflash is exploded at an appropriate miss-distance from a Firefly.
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